Patent GB-998879-A (GB)
Title: Electric signalling circuits
IPC: H04Q3/62
Filed: 1961-11-29
Abstract: 998,879. Automatic exchange systems. WESTERN ELECTRIC CO. Inc. Nov. 29, 1961 [Dec. 6, 19601, No. 42790/61. Heading H4K. A system utilizing reverse-battery signalling comprises at one end of an inter-exchange trunk a pair of relays both of which under normal conditions (call in progress) are released whereby off-normal battery is supplied to the trunk but which are so connected that when during signalling one of them operates the offnormal battery is maintained, that the other one comes up when the first one is released to again maintain the off-normal battery and that when the first one again operates it initiates the slowrelease of the second one whereby normal battery is supplied during the short period when both relays are up together. The invention is applied to a PABX in which a cal subscriber thereof may require to communicate with a toll operator but is prevented from signalling to her directly over the inter-exchange trunk, the arrangement being that by the usual repeated depression of his cradle the subscriber may callin the PABX operator, the latter then initiating the above-mentioned signalling sequence. Setting up a call (incoming).-A calling subscriber is connected to PABX extension 13, say, via a toll switchboard 5, tandem exchange 1 and the PABX switch train 2 and consequently relays SA, SB, SK and CO will be operated assuming that the extension is free. When station 13 answers, relay SJ comes up so as to reverse the battery-ground condition applied via relay SA to conductors 6, 7 and thereby forward bias rectifier D2. Relay P operates and causes the completion of the connection by bringing up relays PA, A, B and SP. Thermal timing relay T comes up slowly in response to P and TA follows, whereby if the line 15 remains looped, the eventual dropping back of T brings up T1 (thus releasing TA) and the trunk circuit is prepared for any subsequent signalling, e.g. operator recall. Also at this time " off-normal " battery and ground are extended via relay A and contacts of one of the signalling relays J and F to the tip and ring conductors 3, 4, respectively. Call transfer.-Station 13 may signal his operator by momentarily operating his cradle switch 22, whereby SJ releases. The consequent battery reversal on conductors 6, 7 brings down P and PA. Thermal relay T comes up and if the cradle switch is not restored within about 0-8 seconds, relay TA follows to initiate release of the connection two seconds later. However, if SJ re-operates within this period P, PA come up and T is released and in addition T1 now drops out since relay T2 came up when PA momentarily fell back. Relay AA is energized and causes ringing current to be applied from source 14 to the line so that both parties are aware that the operator has been signalled and a lamp TL is also operated. The operator inserts her plug associated with cord-circuit 16 into jack TJ and thus brings up sleeve relay S and relay J, which thereby releases AA, T2 and connects the operator to the connection. She may then connect any other PABX extension to the connection using the other end of the cord circuit. Recall of toll operator.-As before, the called extension flashes the PABX operator but in this case informs her that he wishes to speak to the toll operator, whereupon the PABX operator withdraws and inserts her plug into jack TJ a number of times so that a polarized relay TS at the toll switchboard can operate to battery reversal and so flash lamp SL. Just before the cord circuit is connected in response to the flashing by the called extension relay A supplies off-normal battery and ground to trunk 3, 4 and relays J and F are down. Upon insertion of the plug, off-normal battery is maintained but J operates. Removal of the plug causes J to fall back and F to come-up (relays T1 and T2 being down at this time) but again off-normal battery is still connected to the trunk. Reinsertion of the plug operates J and slowrelease relay F stays up temporarily so that normal battery and ground are reverted on the trunk. Relay TS responds and the toll operator's lamp SL is lit. When F relapses, off-normal battery is returned and lamp SL is thus extinguished. This procedure is repeated until the toll operator answers. If the PABX operator does not reinsert her plug within 2À8 seconds relay T operates, followed by TA and T1 so that the signalling relays J and F are disabled. Facilities are provided for calling or called party release.